Bonded nonwoven fabrics and methods of making the same



Feb. 25, 1964 BONDED NONWOVEN FABRICS AND METHODS OF MAKING THE SAME F. H. sExsMm-l 3,122,447

Filed Nov. 21, 1962 INVENTOR V/f/'P MAQ/raw rf/ ATTORNEY United States Patent 3,122,447 BNDED N NWQVEN FABRECS AND METHQDS F MAKHIG l Hts SAME red H. Sexsmith, Montclair, NJ., assigner, by mesne assignments, to lohnson l; .lohnsom New Brunswick, NJ., a corporation of New Jersey Filed Nov. 2l, 1952, Ser. No. 239,151 19 Claims. (fl. 117-140) This invention relates to nonwoven vfabrics produced by bonding together the bers of a loosely assembled web of bers.

This application is a continuation-impart of my copending applications, Serial No. 860,858 and Serial No. 860,679, both led December 21, 1959, now abandoned.

Bonded nonwoven fabrics have been formed hitherto by impregnating, printing or otherwise depositing an adhesive bonding material on a base web predominantly comprising relatively long bers, including those of textile length of from about 1/2 inch 'to about 21/2 tinches or more. Such base webs may be vformed by garnetting, air deposition, carding, etc. Generally speaking, it is the aim of such processes to produce a lsoft fabric with as much strength as possible. Choice of bonding agent is very vimportant in obtaining both softness and strength. Many bonding agents which have obtained importance in the art, such as polyvinyl acetate, have suilicient bonding strength when dry, but lose the greater portion of this strength when the fabric is wetted with water. This wert strength may be defined `as the ability of the fabric to retain part of its dry strength after substantially complete saturation with water. 'Ilhis property of wet strength is very important when the fabric to be used as a towel, in a sanitary napkin, in a disposable diaper yor in any other manner where i-t will become moist or in contact with iiuids.

The softness of nonwoven fabrics may be increased in a number of ways, such as by applying the adhesive intermittently as outlined in U.S. Patents 2,705,687 and 2,705,688, by addition of plasticining agents to the binder as described in U.S. Patent 2,833,283 by after-'treatment with cationic or non-ionic softening agents, etc. However, there is still a major problem with these fabrics, i.e., they lose considerable strength on wetting with water. This is especially true of fabrics after-treated .with softening agents as it appears that 'the softening agents affect the resin-ber bonds. There are bonding agents known which do not lose considerable strength when wetted, such as regenerated cellulose, derivatives of cellulose and chitin, among others. However the aforementioned agents do not lhave all the desirable properties orf the vinyl resin polymers yand copolymers.

Considerable time and elort have been expended on developing techniques for improving the wet strength of bonding agents :for nonwoven fabrics consisting predominantly of cellulose bers. One of the more important developments in this eld has been the 4treatment of the fabrics with aminoaldehyde condensation products such as urea-formaldehyde and melamine-formaldehyde, along with heat and ya catalyst to set the resin and cross-link the cellulose bers. Such treatments, though they increase wet strength, have considerable disadvantages in that they are costly and complicated, reduce the softness of the fabric, and have a detrimental odor due to the formaldehyde release.

It is the purpose of this invention to provide a new method for improving the wet strength of nonwoven fabrics 'formed predominantly from cellulose textilelength bers, bonded with vinyl polymers and copolymers, without harming the softness of such a fabric.

ln 'accordance with this invention, vinyl resimbonded nonwoven fabrics of cellulose, textile-length bers are ob- Frice 2 tained which retain a greater portion of their dry strength after substantially complete saturation with walter. =Fur thenmore, desired, the nonwoven fabrics of the present invention may be after-treated with softening agents without detrimental effects on the fabric strength.

The wet strength of the vinyl resin-bonded nonwoven fabri is increased by adding to the resin, prior to applying the resin Ito the fibrous web, a small quantity of a water-soluble polyallrylene inline. Although the inventive concept will be described with particular reference to a specic polyalkylene imine, for example, polyethylene imine, such -is merely for illustrative purposes and is not to be construed `as limitative of the broader aspects involved.

Polyethylene inline is essentially a linear poly secondary amine. It is prepared by the acid catalyzed polymerization of ethylene irnine according .to `the following reaction:

Polymerization is spontaneous and if uncontrolled, will lead to water-insoluble products which are diicult to apply to textile fabrics. If the polymerization is stopped before water-insolubility occurs, the application of the polymerized ethylene irnine yto the fabric is greatly simplied, and it is such a degree of polymerization which is preferred in accordance with the method for applying polyethylene inline to brous webs.

Polyethylene imine is available in a 50% aqueous solution. Itis a highly viscous liquid with a slight amine-like odor.

The term vinyl resins is meant to include all thermoplastic polymers, copolymers, and mixtures thereof of vinyl monomers such as vinyl chloride, vinyl acetate, styrene, alkyl acrylates, etc.

The vinyl resins generally used in accordance with this invention are available as Water-dispersed resins or latices of nely divided resin particles ranging from 0.1 to 20 microns in diameter suspended in water to about 45% to 55% total solids. The resin dispersions may also contain certain additives such as plasticizers, protective colloids, dyes, emulsiers, ilanleproong agents, mildewproofing agents, stabilizers, etc.

Of particular interest are those dispersions of poly. vinyl acetate containing polyvinyl alcohol as the protective colloid. These dispersions are of excellent mechanical stability and printability, but normally confer water sensitivity to ber assemblies.

The starting layer may be formed by any one of the conventional techniques for depositing or arranging bers in a web or layer. These techniques include carding, garnetting, air-laying and the like. Individual webs or thin layers formed by one or more of these techniques may be laminated to provide a thicker layer for conversion into a fabric. In general, the fibers extend in a plurality of diverse directions in general alignment with the major plane of the fabric, overlapping, intersecting and supporting one another to form an open, porous structure.

When speaking of cellulose bers, it is meant those bers containing predominantly CGI-11005 groupings. Exemplary or" the bers to be used in the starting layer are the natural cellulose bers such as cotton and hemp and the synthetic cellulose fibers, notably rayon, and regenerated cellulose. The fibrous starting'layer should contain at least 50% cellulose bers whether they be natural or synthetic or a combination thereof. The remaining bers in the starting layer may comprise natural bers such as wool, or jute; artificial bers such as cellulose acetate; synthetic fibers such as polyamides, ie.,

3 nylon, pciyesters, i.e., Dacrom acrylics, i.e., Dynel, Acrilan, Orlon, polyolens, i.e., polyethylene, polyvinyl chloride, polyurethane, ctc., alone or in combination with one another.

The fibrous startinfr layer weighs from about 100 grains to about 2,000 grains per square yard and preferably eighs about 200 grains to about 800 grains per square yard.

The resulting fibrous starting layer, regardless of its method of production, is then subjected to at least one of the several types of bonding operations to anchor the individual fibers together to form a self-sustaining web. Some of the better-known methods of bonding are overall impregnation, and printing the web with intermittent or continuous straight or wavy lines or areas of binder extending generally transversely or diagonally across the web and additionally', if desired, along the web.

The desired amount of binder, calculated on a dry addon basis, to be applied to the fibrous starting layer, may range from about 1% to about 103% by weight of the starting web and preferably from about to about 35% by weight of the starting web.

The amount of polyalkylene imine to be added to the resin dispersion may vary from about 1i% to about 21/2 by weight of the resin. The preferred amount of polyalkylene imine to be added is from about 1/2 to about 11/2%l by weight of the resin. lf less than v11% is used, the wet-strength contribution of the polyalkylene inn'ne is substantially negligible. While more than 21/2% polyalkylene imine may be used, the resultant increase in wet strength from the additional polyalkylene imine is also substantially negligible, and such greater amounts may in fact start to decrease the wet strength again. Using more than 11/2% polyalkylene irnine is not economically feasible at the present time.

The effect of increasing the proportions of polyethylene imine in a polyvinyl-acetate resin binder is illustrated in the drawing wherein the ligure is a graph of the ratio of wet strength to dry strength of a fabric weighing approximately 24() gr./yd.2 and containing about 40 gr./yd.2 of binder plotted against the weight percent of polyethylene imine based on the total weight of binder.

The polyethylene imine used is usually a 50% aqueous solution in order to simplify handling and processing techniques. The binder dispersions used are usually dispersions containing 45% to 55% solids with the remainder of the dispersion being water.

The softness of nonwoven fabrics made as described above may be improved by impregnating the fabric with an aqueous solution of a cationic or non-ionic surfaceacting softening agent.

Normally such softening agents are harmful to the bond between the cellulose fiber and the vinyl resin and reduce fabric strength. However, when such softening agents are used in the presence of polyalkylene imine fabric strength is not harmed. It appears as though the polyalkylene imine Orients itself on the outer surfaces of the vinyl resin particles thus forming a protective barrier about the vinyl resin and preventing attack by the cationic or nonionic surface-acting agents on the vinyl resin to cellulose fiber bond.

Illustrative of the cationic softeners are the long-chain quaternary ammonium salt compounds containing one or two ahryl groups, said alkyl groups containing from 8 to 18 carbon atoms. Such compounds are sold by the Armour Chemical Company under the trade name Arquad. ther quaternary compounds which may be used are the quaternary ammonium salts of amino amides such as the quaternized guanidinium salts containing long straight aliphatic chains, said chains containing from 8 to 1S carbon atoms. Such compounds are sold by the Ciba Company under the trade name Sapamine Some non-quaternary cationic softeners which may be used in accordance with this invention are the long-chain cornpounds containing primary, secondary or tertiary amino groups. illustrative of this group of compounds are the condensates of polyamides such as diethyltriamine and its analogs with less than 1 equivalent of a fatty acid such as stearic acid. These compounds are also sold by the Ciba Company under the trade name Sapamine. Other cationic compounds which may be used are the long-chain compounds containing thiourea groups instead of the amino or quaternary groups previously mentioned. Illustrative of the nonionic softening agents are the amides of the aliphatic diamines and polyarnines, the polyethenoxy derivatives of higher fatty alkylolarnides and higher fatty acids and the glycerides of fatty acids.

Probably the most important softening agents of the `above-described group are the monoalkyl ltrirnethyl ammonium chlorides and the dialkyl dimethyl ammonium chlonides. These compounds not only soften the fabric but have a considerable degree of bacteriostatic action which is considerably important for sanitary and surgical nonwoven-fabric products.

Such compounds may be represented by the following where R is an alkyl group containing 8 to 18 carbon atoms; a is selected from integers 2 and 3; b is selected from the integers l and 2; and X is a halogen.

e amount of softening agent used will, of course, depend on the softness desired in the end product and the exact type softener used. Generally speaking, the amount of softening agent used in accordance with this invention may be ias low as about 1/2% or as much as `about 5% based on 'the weight of the bonded fabric. We prefer to keep the amount of softening agent Iapplied between .about v1/2. to about 11/2 based on the weight of the ybonded fabric. If less than 1/2 is used, the increase in fabric softness is substantially negligible. While more than 5% of the softening agent may be used, it ris not economically feasible lat the present time as the increased benefit-s do not warrant the consequent increased cost. The softener is generally applied by forming an emulsion of the `softener in water and then impregnat-ing the web with the emulsion tto give the desiredpickup so that when the fabric lis dried, it contains the desired percentage of softening agent required for the end-use product.

The invention will be further illustrated in greater detail by the lfollowirv7 specific examples. It should be understood, however, that although these examples may describe in particular detail some of the more specific features of vthe invention, they are given prima-nily for purposes of illustration, and the invention, in its broader aspects, is not to be construed as limited thereto.

Example I A polyvinyl-acetate binder is prepared as follows: 295 grams of la plastioizer, dibutoxyethyl phthalate, is yadded to 3,175 grams of a homopolymer polyvinyl-acetate resin dispersion of 55% solids. Ammonia is then )added to raise the pH of the dispersion to approximately 7, and at this point, a small amount of antifoaming ugent sold as Anti Foam A, by the Dow-Corning Co., is addedto aid in processing. This dispersion is then diluted with 30 grams of water. No polyalkylene imine is included.

This binder dispersion is printed on a 2O0-gr./yd.2, %-rayon, `of 11/2 denier, Z-inch ber length, carded web in an intermittent pattern consisting of 4 horizontal wavy lines per inch. The print lines run the width of the web iand cover less than 50% of the total surface area of the web. The total `amount of binder picked up by the web on -a dry-'add-on basis, is between 35 to 40 gr./yd.2.

The dry strength and the -wet strength of this fabric is then tested. This is done by nandomly cutting 12 samples 'Ille fabric is then dried at approximately the web and cover less than 30% of the total surface area of the web. The total amount of binder picked up by the web is about 4S gr./yd.2. The fabric is then dried at approximately 240 F.

This fabric is then impregnated ywith an aqueous solution containing about of a dialkyl dimethyl-ammonium chloride sold by the Armour Chemical Co. under the trade name Arquad ZET-75, to a moisture pickup of 150% of the weight of the fabric. The aikyl groupings in Arquad 2HT-75 are approximateiy 25% hexadecyl groups and 75% octadecyl groups. This fabric is then dried at approximately 240 F. so that the final fabric contains approximately 1% of the softening agent, based on the final weight of the fabric.

This fabric is then tested for its strength when dry and when saturated with water. The fabric is then also tested for softness or fliexural rigidity. The strength of the fabric is determined as described in Example I. The average dry strength of the fabric is 50. The average Wet strength is determined as about 22.6. The ratio of wet strength to dry strength is approximately 45%.

The softness of the fabric is also determined. The softness or flexural rigidity of a fabric is determined by cutting four 81/2-inch-square samples randomly from the fabric. Each sample is then tested on a Thwing- Albert iiandle-G-Meter, an instrument designed and manufactured by the Thwing-Albert instrument Co. of Philadelphia, Pa., for testing the softness of flexural resistance of fabrics. A metal bar bends the fabric, and the resistance to iiex is determined in milliamperes, which is converted to a softness figure. The average softness of this fabric, as determined by this machine, is about 88.

Example XII A binder dispersion is prepared as described in Example XI. No polyalkylene imine is added to the dispersion, but it is applied directly to the base web. This fabric is not aftertreated with a chemical-softening agent, but is tested for dry strength and Wet strength and also for softness as outlined in Example XI. The dry strength of the fabric is 54.0, and the wet strength is only 16.4, giving a ratio of wet to dry strength of only 30%. The softness of ythis fabric is poor, giving a reading of only 79 on the Thwing-Albert Handle-O-Meter.

Example XIII A portion of the fabric made as described in Example II is aftertreated with 2/s% aqueous solution of a dialkyl dimethyl ammonium chloride sold by the Armour Chemical Co. under the trade name of Arquad 2HT-75. The fabric is treated so as to pick up 1% of the softening agent based on the dry weight of the fabric. This fabric is also tested for dry and wet strength, and softness as described in Example XI. The softness of the fabric is excellent, giving a reading on the Thwing-Albert Handle- ()Meter of 88; but the dry strength is only 42.0, and the wet strength is just as poor being only 15.0. The ratio of wet strength to dry strength of the fabric is about 35%.

Example XIV The procedure of Example Xi is followed except that in the rst instance the aftertreatment of the bonded web with the chemical-softening agent is conducted so as to allow for only 75 moisture pickup based on the weight of the final weight of the fabric. In the second instance the moisture pickup is controlled at 225% so that the final fabric contains 11/2% of the softening agent. In both cases results comparable to those obtained in Example XI are obtained.

Example XV The procedure of Example XI is followed, except that a cationic softener, of the high-molecular-weight amineacetate type sold by the Refined Products Co., under the trade name Perma-Par RG, is substituted for the dialkyl dimethylammonium chloride. Similar results are obtained. The same procedure is again followed, but this time a long-chain compound containing secondary and tertiary arnino groups in its structure, sold under the trade name Sapamine by the Ciba Co., Inc., is substituted for the dialkyl dimethyl ammonium chloride, and again similar results are obtained.

Example XVI The procedure of Example XI is followed, except that the binder dispersion is polyvinyl chloride rather than polyvinyl acetate. Similar results are obtained.

Since many variations and modifications may be made in practicing this invention without departing from the spirit and scope of the invention, it is intended that all matter contained in the above description shall be interpreted as descriptive and not in a limiting sense.

It is also to be understood that the inventive concept is not to be limited except by the scope of the appended claims.

What is claimed is:

1. A soft and strong nonwoven fabric comprising a loosely assembled web of overlapping, intersecting bers, of which at least about 50% by Weight are cellulose fibers and a binder material distributed throughout said web in adhesive bonding Contact with the fibers therein, said binder material possessing wet strength and cornprising a Vinyl acetate resin polymer and, dispersed in said vinyl acetate resin polymer as a wet strength improver therefor, a polyalkylene imine in an amount of from about to about 21/2 by weight, based on the weight of said vinyl acetate resin polymer.

2. A soft and strong nonwoven fabric comprising a loosely assembled web of overlapping, intersecting fibers, of which at least about 50% by weight are cellulose fibers, and a binder material distributed throughout said web in spaced areas covering less than about 35% of the surface of the web and in adhesive bonding relationship with the iibers therein, said binder material possessing wet strength and comprising a vinyl acetate resin polymer and, dispersed in said vinyl acetate resin polymer as a wet strength improve therefor, a polyalkylene imine in an amount of from about Mt to about 21/2 by weight, based on the weight of said Vinyl acetate resin polymer.

3. A soft and strong nonwoven fabric comprising a looseiy assembled web of overlapping, intersecting fibers, of which at least about 50% by weight are cellulose fibers, and a binder material distributed throughout said web in adhesive bonding contact with the fibers therein; said binder material possessing wet strength and comprising a vinyl acetate resin polymer and, dispersed in said vinyl acetate resin polymer as a Wet strength improver therefor, a polyethylene imine in an amount of from about l/J.% to about 21/2% by Weight, based on the weight of said vinyl acetate resin polymer.

4. A soft and strong nonwoven fabric comprising a loosely assembled web of overlapping, intersecting fibers, of which atleast about 50% by weight are cellulose fibers, and a binder material distributed throughout said web in spaced areas covering less than about 35% of the surface of the web and in adhesive bonding relationship with the fibers therein, said binder material possessing wet strength and comprising a vinyl acetate resin polymer and, dispersed in said vinyl acetate resin polymer as a wet strength improver therefor, a polyethylene imine in an amount of from about to about 21/2% by weight, based on the weight of said vinyl acetate resin polymer.

5. A soft and stronor nonwoven fabric comprising a loosely assembied web of overlapping, intersecting fibers, of which at least about 50% by Weight are cellulose fibers, and from about 1% to about 100% by weight of the web of a binder material distributed throughout said web in adhesive bonding Contact with the fibers therein, said binder material possessing wet strength and comprising a vinyl acetate resin polymer and, dispersed in said vinyl V ing tensile strengths of fabrics. Q Vthe fabric until it breaks. When determining the wet f1 I `vof the fabric, a similar procedure is followed from the fabrics. The samples are 1" x 6" in size with the 6 dimension being the machine direction. The dry strength of the fabric lis determined by placing the 4 samples in the jaws of a Scott-1PA Tester, an instrument designed and manufacture-d specioally for determining the strength of `-a fabnic by Scott Testers, Inc. of Providence, R.I. A force is :applied to each end of the sample until it breaks. When determining the wet strength of the fabric, a similar procedure is followed with the exception that before placing the fabric samples in the jaws of the Scott-IP-4 Tester, each sample is saturated with 3 cc. of water Iand allowed Ito dry for 30 seconds on a piece of kraft paper. This procedure is followed with 3 groups of 4 samples each in order to obtain an average result for the strength of the fabric. The 'wet strength of Ithis fabric is only 28% of its dry strength.

Example II A binder dispersion is prepared as described in Example 'L To this binder ydispersion approximately 14 grams of 50% aqueous polyethylene imine is added. The polyethylene imine has a molecular weight ranging from 30,000 to 40,000. This binder dispersion is applied in a similar manner and to a similar web I'as that described in Example I. This fabric is lthen tested for dry strength and wet lstrength as described in Example I. The wet strength of this :fabric is increased `t-o 37.8% of the dry strength. .'Iihis degree of wet strength in a fabric is acceptable for many products, though there are certain uses for Ia nonwoven fabric where it Iis `desired to attain a higher value for the Wet strength such as in the cover tfor an :absorbent medium.

Example III A binder dispersion is prepared las described in Example I. To this binder dispersion approximately 35 grams of 50% yaqueous polyethylene imine is added. The binder dispersion is then applied to a base web -in the manner 'as described yin Example I. This fabric is then tested for dry strength and wet strength as described in Example I. The wet strength of the fabric is 47.6% of the dry strength.

Example 1V A polyvinyhacetate binder is prepared as follows: 39 pounds of a plasticizerr, dibutoxyethyl phthalate, is added to 420 pounds 'of a homopolyrner polyvinyl acetate-resin dispersion of 55% solids. Three pounds of ammonium hydroxide is then added to the dispersion fand a small amount of an lantifoaming agent, as well as other conventional Iagents to aid in processing. This dispersion -is then diluted with 4 pounds of Water. Nopolyethylene imine is included.

This binder dispersion is then printed on a 200-gr./yd.2, 100%-rayon, of l-/z denier and 2-inch fiber length, carded web in an intermittent pattern of ring-like areas as described in U.S. Patent 2,705,688. The total amount of binder picked up by the web is approximately 35 to 40 gr./yd.2. The fabric is then dried at approximately 240 F.

A water solution containing 1% of a 50% aqueous polyethylene imine is then separately padded on the aforementioned fabric to saturate the fabric, approximately 100% pick-up. The fabric is then air-dried and tested to determine its wet strength and dry strength.

The strength of the fabric is tested by randomly taking 5 samples from the fabric. The samples are 1" x 6 in size, with the 6" dimension being the machine direction of the fabric. The dry strength of the fabric is determined by placing each of the samples in the jaws of an Insco Tensile Tester, an instrument manufactured by the Insco Co. of Natick, Mass., specifically for test- A force is applied to except immediately prior to placing the samples in the lnsco Tensile Tester, they are saturated with 3 cc. of water and allowed to dry for 30 seconds on kraft paper. This procedure is followed with each individual sample in order to obtain an average result for the fabric tested. The wet strength of this fabric is only 10% of its dry strength.

Samples of this same basic fabric without the polyethylene imine aftertreatrnent are tested in the same manner in order to determine the wet strength and dry strength of the fabric. The wet strength of the fabric Without the polyethylene imine aftertreatment is only 10% of its dry strength.

Example V Example VI A binder dispersion is prepared as described in ample I. To this binder dispersion approximately 55 grams of 50% aqueous polyethylene imine is added. This binder dispersion is applied in a similar manner and to a similar web as that described in Example l. Comparative results are obtained.

Example VII The procedure of Example lll is followed except that approximately 35 grams of 50% aqueous polyrnethylene imine is added to the binder dispersion rather than polyethylene imine. Similar results to those obtained in Example lll are obtained.

Example VIII The procedure of Example lll is followed except that polypropylene imine is added to the binder dispersion instead of polyethylene imine. Similar results are obtained.

Example IX The procedure of Example I is followed except that the binder dispersion is a copolymer of polyvinyl acetate and polyvinyl chloride rather than the homopolymer vinyl acetate. Similar results are obtained.

Example X The procedure of Example I is followed except that the fibrous starting layer contains 50% rayon and 50% nylon. Similar results are obtained.

Example XI A polyvinyl-acetate binder is prepared as follows: 15 grams of a plasticizer, dibutoxyethyl phthalate, is added to 160 grams of a homopolyrner polyvinyl-acetate resin dispersion sold by E. I. du Pont de Nemours & Co. Inc. under the trade name of Elvacet 81-900. The pH of the dispersion is raised to 7, and a small amount of antifoaming agent sold as Anti-Foam A by the Dow- Corning Co., as well as other conventional agents, are added to aid in processing. To this binder dispersion is added 1.5 grams of 50% aqueous polyethylene imine. The polyethylene imine has a molecular weight ranging from 30,000 to 40,000.

This binder dispersion is printed 0n a 200 gr./yd.2, rayon, of 11/2 denier, Z-inch fiber length, carded web in an intermittent pattern consisting of 4 horizontal wavy lines per inch. The print lines run the width of 79 acetate resin polymer asa wet 'strength improver therefor, a polyethylene imine in an amount of from about to about 21/2'% by weight, based on the weight of said vinyl acetate resin polymer.

6. A soft and strong nonwoven fabric comprising a loosely assembled Web of overlapping, intersecting fibers, of which at least about 50% by weight are cellulose fibers, and from about to about 35 by weight of the web of a binder material distributed throughout said web in adhesive bonding contact with the fibers therein, said binder material possessing wet strength and comprising a vinyl acetate resin polymer and, dispersed in said vinyl acetate resin polymer as a wet strength improver therefor, a polyalkylene imine in an amount of from about 1/4% to about 21/z% by weight, based on the weight of said vinyl acetate resin polymer.

7. A soft and strong nonwoven fabric comprising aloosely assembled web of overlapping, intersecting fibers, of which at least about 50% by weight are cellulose fibers, and a binder material distributed throughout said web in adhesive bonding contact with the fibers therein; said binder material possessing wet strength and comprising a vinyl acetate resin polymer and, dispersed in said vinyl acetate resin polymer as a wet strength improver therefor, a polyethylene imine in an amount of from about 1/2% to about l1/2% by weight, based on the weight of said vinyl acetate resin polymer.

8. A soft and strong nonwoven fabric comprising a loosely assembled web of overlapping, intersecting fibers of which at least about 50% by weight are natural vegetable fibers, and a binder material distributed throughout said web in adhesive bonding contact with the fibers therein, said binder material possessing wet strength and comprising a vinyl acetate resin polymer and, dispersed in said vinyl acetate resin polymer as a wet strength irnprover therefor, a polyalkylene imine in an amount of from about to about 21/2 by weight, based on the weight of said vinyl acetate resin polymer.

9. A soft and strong nonwoven fabric comprising a loosely assembled web of overlapping, intersecting fibers of which at least about 50% by weight are regenerated cellulose fibers, and a binder material distributed throughout said web in adhesive bonding contact with the fibers therein, said binder material possessing wet strength and comprising a vinyl acetate resin polymer and, dispersed in said vinyl acetate resin polymer as a wet strength improver therefor, a polyalkylene imine in an amount of from about to about 21/2% by weight, based on the weight of said vinyl acetate resin polymer.

l0. A soft and strong nonwoven fabric comprising a loosely assembled web of overlapping, intersecting fibers of which at least about 50% by weight are cotton fibers, and a binder material distributed throughout said web in adhesive bonding contact with the fibers therein, said binder material possessing wet strength and comprising a vinyl acetate resin polymer and, dispersed in said vinyl acetate resin polymer as a wet strength improver therefor, a polyalkylene imine in an amount of from about 11% to about 21/2% by weight, based on the weight of said vinyl acetate resin polymer.

ll. A soft and strong nonwoven fabric comprising a loosely assembled web of overlapping, intersecting fibers of which at least about 50% by weight are viscose rayon lfibers and a binder material distributed throuhgout said web in adhesive bonding contact with the fibers therein, said binder material possessing wet strength and comprising a vinyl acetate resin polymer and, dispersed in said vinyl acetate resin polymer as a wet strength improver therefor, a polyalkylene imine -iu au amount of from about 1A% to about 21/2% by weight, based on the weight of said vinyl acetate resin polymer.

i12. A soft, strong nonwoven fabric comprising a loosely assembled web of overlapping, intersecting fibers, of which at least about 50% by weight are viscose rayon fibers, and a binder material distributed throughout said web in spaced areas covering less than about 35% of the surface of the web and in adhesive bonding relationship with the fibers therein, said binder material possessing wet strength and comprising a vinyl acetate resin polymer and, dispersed in said vinyl acetate resin polymer as a wet strength improver therefor, a polyethylene `imine in an amount of from about 1/2% to about 11/2% by weight, based on the weight of said vinyl acetate resin polymer.

13. A soft and strong nonwoven fabric comprising a loosely assembled web of overlapping, intersecting fibers, of which at least 510% by weight are cellulose fibers, a binder material distributed throughout said web in spaced areas covering less than about 35% of the surface of the web and in adhesive bonding relationship with the fibers therein, the binder material possessing wet strength and comprising a vinyl resin polymer, and dispersed in said vinyl resin polymer as a wet strength improver therefor, a polyalkylene imine in an amount of from about to about 21/2 by Weight, based on the weight of the vinyl resin polymer, and from about 1/2% to about 5% based on the weight of the fabric of a chemical softening agent selected from the group consisting of cationic surfaceactive agents and nonionic surface-active agents distributed substantially uniformly throughout they bonded fibrous web.

14. A soft and strong nonwoven fabric comprising a loosely assembled web of overlapping, intersecting bers, of which at least 50% by `weight are cellulose bers, a binder material distributed throughout said web in spaced areas covering less than about 35 of the surface of the web and in adhesive bonding relationship with the fibers therein, the binder material possessing wet strength and comprising a vinyl acetate resin polymer, and dispersed in said vinyl acetate resin polymer as a wet strength improver therefor, a polyethylene imine in an amount of from about 1A to about 2'1/2% by weight, based on the weight of the vinyl acetate resin polymer, and from about 1/2% to about 5% based on the weight of the fabric of a chemical softening agent selected yfrom the group consisting of cationic surface-active agents and nonionic surfaceactive agents distributed substantially uniformly throughout the bonded fibrous web.

l5. A soft and strong nonwoven fabric comprising a loosely assembled web of overlapping, intersecting fibers, of which at -least 50% by Weight are celluose fibers, a binder material distributed throughout said web in spaced areas covering less than about 35% of the surface of the web and in adhesive bonding relationship with the fibers therein, the binder material possessing wet strength and -comprising a vinyl resin polymer, and dispersed in said vinyl resin polymer as a =wet strength improver therefor, a polyalkylene imine in an amount of from about 1t% to about 21/2% by weight, based on the weight of the vinyl resin polymer, and from about '1/2% to about 5% based on the Weight of the fabric of a dialkyl dimethyl ammonium chloride, said alkyl groups containing 8 to 18 carbon atoms, distributed substantially uniformly throughout the bonded fibrous web.

16. A soft and strong nonwoven fabric comprising a loosely assembled web of overlapping, intersecting fibers, of which at least 50% by weight are cellulose fibers, a binder material distributed throughout said web in spaced areas covering less than about 35 of the sur-face of the web and in adhesive bonding relationship with the fibers therein, the binder material possessing wet strength and comprising a vinyl acetate resin polymer, and dispersed in said vinyl acetate resin polymer as a wet strength improver therefor, a polyethylene imine in an amount of from about to about 21/2% by weight, based on the weight of the vinyl acetate resin polymer, and from about 1/2 to about 5% based on the weight of the fabric of dialkyl dimethyl ammonium chloride, said alkyl groups containing 8 to 18 carbon atoms, distributed substantially uniformly throughout the bonded fibrous web.

17. A method of forming a soft and strong nonwoven fabric having improved Wet strength which comprises bonding a loosely assembled web of overlapping, intersecting ibers of which at least about 50% by weight are cellulose bers with a binder material comprising a dispersion of a vinyl acetate resin polymer, and, dispersed in said vinyl acetate resin polymer dispersion as a wet strength improver therefor, a polyalkylene imine in an amount of from about to about 21/z% by Weight, based on the weight of the vinyl acetate resin polymer, said polyalkylene imine being dispersed in said vinyl acetate resin polymer dispersion prior to its application to said web.

18. A method of forming a soit and strong nonwoven fabric having improved wet strength which comprisesv bonding a loosely assembled web of overlapping, intersecting ii'bers of which at least about 50% by weight are cellulose bers with a binder material comprising a dispersion of a Vinyl acetate resin polymer and, dispersed in said vinyl acetate resin polymer dispersion as a Wet strength improver therefor, a polyethylene imine in an amount of from about to about 21/2% by weight, based on the Weight ofthe vinyl acetate resin polymer, said polyethylene imine being dispersed in said vinyl acetate resin polymer dispersion prior to its application to said web.

19. A lmethod of forming a soft and strong nonwoven fabric having improved wet strength and softness which comprises bonding a loosely assembly web of overlapping and intersecting fibers, of which at least 50% by Weight are cellulose fibers, in a pattern of spaced binder are-as covering less than about 35% of the surface of said web, with a binder material comprising a dispersion of a vinyl resin polymer and dispersed in said vinyl resin polymer as a wet strength improver therefor, a polyalkylene imine in the amount of -from about to about 21/2% by weight, based on the weight of the vinyl resin polymer, said polyalkylene imine :being dispersed in said vinyl resin polymer dispersion prior to fthe application of said dispersion to the web, and applying to said bonded ibrous web vfrom about 1/2 vto about 5% of a chemical softening agent selected from the group consisting of cationic surface-active agents and nonionic surface-active agents and drying the same.

References Cited in the file of this patent UNITED STATES PATENTS 2,286,726 `Gordon June'16, 1942 2,343,093 Smith Feb. 29, 1944 2,470,042 McLean May 10,1949V 2,677,681 Gill May 4, 1954 2,705,688 INess IApr. 5, 1955 2,833,283 Spahr et al May 6, 1958 2,912,350 Videen et al Nov. l0, 1959 2,940,889 Justice lune 14, 1960 

17. A METHOD OF FORMING A SOFT AND STRONG NONWOVEN FABRIC HAVING IMPROVED WET STRENGTH WHICH COMPRISES BONDING A LOOSELY ASSEMBLED WEB OF OVERLAPPING, INTERSECTING FIBERS OF WHICH AT LEAST ABOUT 50% BY WEIGHT ARE CELLULOSE FIBERS WITH A BINDER MATERIAL COMPRISING A DISPERSION OF A VINYL ACETATE RESIN POLYMER, AND, DISPERSED IN SAID VINYL ACETATE RESIN POLYMER DISPERSION AS A WET STRENGTH IMPROVER THEREFOR, A POLYALKYLENE IMINE IN AN 